\section{Acknowledgments} 

We would like to thank the following people for providing information
about breakpoints, tracing, and stepping in various \commonlisp{}
implementations: Martin Simmons (LispWorks), Michał ``phoe'' Herda
(CCL), Alex Wood (Clasp), Daniel Kochmański (ECL), Duane Rettig
(Allegro).

We would like to thank Frode Fjeld for giving feedback on early
versions of this paper, and for suggesting the use of multiple entry
points for each function.

We would like to thank Michael Raskin for suggesting that two versions
of each function should be provided, thereby making it unnecessary for
the programmer to choose the level of debugging.
